1. Field of the Invention
The invention pertains to an automated system, and to a corresponding method, for testing electro-optic modules.
2. Description of the Related Art
Those engaged in the manufacture and use of communication systems, e.g., systems for communicating voice, video and/or data, have become increasingly interested in using fiber optic cables as transmission media in such systems. This interest is stimulated by the fact that the potential bandwidth (or information-carrying capacity) of optical fibers is extremely high. In addition, communication systems employing fiber optic cables are resistant to electromagnetic interference, which sometimes plagues systems employing electrical cables as transmission media. Moreover, communication systems employing fiber optic cables are considered more secure than systems employing electrical cables because it is generally more difficult for unauthorized personnel to tap or access a fiber optic cable without being detected.
An exemplary communication system employing a fiber optic cable as a transmission medium is one which includes, for example, two or more computers, with each adjacent pair of computers being interconnected by a fiber optic cable which includes two optical fibers, i.e., a transmit optical fiber and a receive optical fiber. Obviously, each computer generates and receives information, i.e., data, in electrical form. Consequently, each computer is also provided with an electro-optic module, typically mounted on a printed circuit board or printed circuit card of the computer, which converts the electrical signals generated by the computer into optical signals, which are transmitted to the adjacent computer via the transmit optical fiber. In addition, the electro-optic module converts optical signals communicated to the computer via the receive optical fiber into corresponding electrical signals.
An electro-optic module, of the type referred to above, necessarily includes an electro-optic transmitter and an electro-optic receiver. That is, such a module typically includes a housing containing a transmitter optical subassembly (TOSA), a receiver optical subassembly (ROSA) and a pinned ceramic substrate bearing a number of semiconductor integrated circuit devices. Certain of these integrated circuit devices perform transmitter-related functions (and are hereinafter denoted the transmitter ICs) and certain of these integrated circuit devices perform receiver-related functions (and are hereinafter denoted the receiver ICs). The TOSA, which is electrically connected to the transmitter ICs, includes an electro-optic transducer, such as a semiconductor laser or a light-emitting diode (LED), which serves to convert the digital electrical signals generated by the transmitter ICs into corresponding digital optical signals. It is the combination of the TOSA and transmitter ICs which constitutes the transmitter of the module. Similarly, the ROSA, which is electrically connected to the receiver ICs, includes an electro-optic transducer, such as a PIN photodiode, which serves to convert received digital optical signals into corresponding digital electrical signals communicated to the receiver ICs. It is the combination of the ROSA and receiver ICs which constitutes the receiver of the module.
The electrical data communicated by a computer to its electro- optic module is typically communicated in parallel form, whereas the transmitter of the electro-optic module is only capable of receiving electrical data, and producing corresponding optical data, in serial form. Consequently, the printed circuit board or printed circuit card on which the module is mounted usually includes a so-called serializer semiconductor integrated circuit device, which serves to convert parallel electrical data into serial electrical data. Similarly, the receiver of the electro-optic module is only capable of receiving optical data in serial form, and of converting it into corresponding serial electrical data, whereas this electrical data is to be communicated to the computer in parallel form. Therefore, the printed circuit board or printed circuit card on which the module is mounted usually includes a so-called deserializer semiconductor integrated circuit device, which serves to convert serial data into parallel data.
A communication system employing fiber optic cables and electro-optic modules can only operate effectively if its components, including its electro-optic modules, operate in conformity with corresponding operating specifications. Therefore, it has become important to test electro-optic modules to make sure that these modules conform to these specifications, and to detect and correct errors in manufacturing processes which lead to non-conformities in the modules.
The operating specifications, referred to above, typically impose limitations on certain parameters which characterize the performance of a transmitter and of a receiver of an electro-optic module. For example, the transmitter is usually characterized by parameters such as transmitter average power, transmitter rise/fall time, transmitter extinction ratio, transmitter duty cycle distortion tortion and transmitter data dependent jitter, all of which parameters are defined below in connection with the present invention. In addition, the receiver is usually characterized by parameters such as receiver sensitivity, receiver pulse width distortion, receiver signal detect threshold and receiver signal detect assert/deassert times, which parameters are also defined below. Significantly, until the present invention, the tests devised to measure these parameters have all been manual and have therefore required an inordinate amount of time to perform. For example, if one were to manually measure all of the parameters listed above, then this would typically require several hours. As a consequence, it has thus far been impractical and prohibitively expensive, for example, for a manufacturer of electro-optic modules to test each and every electro-optic module manufactured by him, or even a statistically significant fraction of such modules, in order to weed out those which fail to conform to the corresponding specification.
To illustrate the above point, it should be noted that in order to measure, for example, receiver sensitivity, using conventional manual techniques, it is first necessary to manually determine the corresponding eye pattern. That is, a digital electrical serial pattern generator (a type of digital signal generator), having an internal clock, is electrically connected to a semiconductor laser or LED which is optically connected to the receiver under test via an optical fiber. The output of the receiver is, in turn, electrically connected to a digital oscilloscope. In addition, the output of the internal clock of the serial pattern generator is electrically connected, through a manually adjustable time delay unit, to the same digital oscilloscope. The serial pattern generator is then used to transmit a pseudo-random digital electrical signal to the semiconductor laser or LED, at a transmission speed (bit rate) to be used in the corresponding communication system. The semiconductor laser or LED produces a corresponding pseudo-random digital optical signal, which is communicated to the receiver via the optical fiber. In addition, the resulting pseudo-random digital electrical signal generated by the receiver is communicated to the digital oscilloscope, which is triggered by the square -wave clock pulses generated by the internal clock of the serial pattern generator. The transmission speed (bit rate) of these square wave pulses is, of course, the same as that of the pseudo-random digital electrical signal.
As each clock pulse triggers the digital oscilloscope, that portion of the pseudo-random digital electrical signal which is produced by the receiver and is subsequent to the triggering pulse is displayed on the oscilloscope and superimposed on previously displayed portions. By adjusting the oscilloscope to display only a segment of the superimposed digital signals corresponding to the width of a single electrical pulse, a pattern of voltage crossings, like that shown in FIG. 1, is produced. This pattern, called an eye pattern, depicts the number of voltage changes associated with the pseudo-random digital electrical signal produced by the receiver. It should be noted that the width of the eye pattern is related to the time interval during which each pulse in the pseudo-random digital signal being produced by the receiver may be sampled without error, i.e., the wider the eye pattern, the longer the time interval, and vice versa. This is important in connection with the deserializer semiconductor integrated circuit device mounted on the printed circuit board or printed circuit card because this device is typically only capable of sampling the electrical pulses produced by the receiver over a particular time interval and, to avoid errors, the width of the eye pattern should be equal to or greater than this deserializer time interval.
When considering receiver sensitivity, it should be understood that this term denotes the average power of the weakest optical signal the receiver can detect and maintain a specified bit error rate (BER). Thus, receiver sensitivity could, in principle, be measured by adding a digital electrical serial pattern comparator to the apparatus described above, which comparator is capable of comparing the pseudo-random digital electrical signal produced by the receiver to that generated by the serial pattern generator, and counting the number of bits which are in error, while varying optical power. That is, one could connect the electrical output of the receiver to the serial pattern comparator, connect the electrical output of the internal clock of the serial pattern generator to the serial pattern comparator and manually adjust the time delay unit to the setting corresponding to the center of the eye pattern. Then, at a specified average optical power of the laser or LED, one could accumulate a sufficient number of bits and count the corresponding number of bits which are in error. If the desired BER is not achieved, one could then increase or decrease the optical power and repeat the above procedure. However, if it is necessary to achieve a BER of, for example, 10**-12 or 10**-15 (as is now required in many computer systems), then one must necessarily accumulate at least 10**12 or 10**15 bits, which could easily take hours or days.
To reduce the amount of time required to measure receiver sensitivity, it is conventionally assumed that the noise in the receiver is Gaussian in nature and, for a fixed temperature, is constant. As a consequence, it follows that a signal-to-noise parameter, Q, associated with the receiver increases linearly with received average optical power, P (expressed in milliwatts), where .vertline.receiver decision threshold level-mean of signal level .vertline. EQU Q=standard deviation of signal level (1)
Stated alternatively, it follows that the log (base 10) of Q is linearly proportional to P, expressed in decibels (dB), as depicted in FIG. 2. It also follows from the above assumptions, and it has been believed, that the slope of logQ versus P (expressed in dB) is equal to 0.0946 dB**-1 for values of logQ ranging from about 0.677 to about 0.847. Moreover, it follows that BER is related to Q through the complementary error function, i.e., ##EQU1## Thus, if BER has been measured at a particular value of P, one can then calculate the corresponding value of Q from Equation (2). Furthermore, with this one data point, one can then obtain a plot of logQ versus P (expressed in dB) by extrapolation, using the above-mentioned slope.
Conventionally, to save time, when manually measuring receiver sensitivity, one uses the above-described apparatus to measure the BER corresponding to a relatively low average optical power, P. Because P is relatively low, it follows that the BER will be relatively high, e.g., 10**-8, and therefore it is only necessary to accumulate a relatively small number of bits, e.g., 10**8, which requires a relatively short period of time. Using Equation (2), it is then conventional to calculate the corresponding value of Q and, based upon the assumption that the slope of logQ versus P (in dB) is 0.0946 dB**-1, obtain a plot of logQ versus P (in dB) for the receiver under test. To determine the power level needed to achieve a desired BER, one then solves Equation (2) for the corresponding value of Q. Using this value of Q, it is then conventional to use the plot of logQ versus P (in dB) to determine the corresponding value of P.
Using the time-saving procedure described above, a manual measurement of receiver sensitivity still requires about thirty (30) minutes. Unfortunately, this is far too long to permit each and every electro-optic module to be tested. In fact, this is far too long to permit even a statistically significant number of modules to be tested.
Thus, those engaged in the development and manufacture of fiber optic communication systems, in general, and of electro-optic modules, in particular, have long sought, thus far without success, systems and methods for testing electro-optic modules which require relatively short testing times.